1. Field of the Invention
The present invention relates to a carbon felt impregnated with inorganic particles and a method for producing the same. More specifically, the present invention relates to a carbon felt that can be used together with sulfur in a cathode of a sodium-sulfur (Na—S) battery, and a method for producing the impregnated carbon felt.
2. Description of the Related Art
A general sodium-sulfur (Na—S) battery is a sealed secondary battery for large capacity power storage that is operated at a high temperature of 300 to 350° C. The Na—S battery has the advantages of being free from self-discharge and high charge-discharge efficiency. The Na—S battery includes a Na anode, a sulfur cathode, and a solid electrolyte (beta alumina) separating the two electrodes. Only Na+ ions selectively pass through the electrolyte. Since sulfur is an insulator, the sulfur (S) electrode uses a felt impregnated with sulfur. The felt is composed of carbon fibers through which electrons created as a result of a chemical reaction between sulfur and sodium can move.
The carbon felt is a mat composed of carbon fibers whose diameter is from several microns to a dozen of microns. The carbon felt has a porosity of about 90%. The sulfur electrode is produced by impregnating the carbon felt with sulfur. In the sulfur electrode, Na+ ions are non-uniformly distributed, which causes many problems.
Particularly, since slow migration of sodium polysulfides causes different degrees of polarization, sulfur failing to participate in the reaction at a proper time or sodium polysulfides are isolated and aggregated, causing the problem that the amount of recirculating electric current may vary. In order to solve this problem, the migration velocity of Na+ ions needs to be increased. Many methods have been proposed to increase the migration velocity of Na+ ions. For example, forced convection of Na ions was proposed but is impractical. High operating temperatures of batteries undesirably accelerates the corrosion of the batteries. A reduction in the viscosity of sulfur has never been reported before. Increased mobility of Na+ ions leads to an improvement in the performance of Na—S batteries. Thus, research is currently being conducted to increase the mobility of Na+ ions.
Carbon surface tends to be wet with sulfur but is not easily wet with sodium polysulfides. In contrast, alumina surface tends to be wet with sodium polysulfides but has poor wettability with sulfur. Accordingly, the use of a carbon felt attached with alumina will increase the wicking of sodium polysulfides, thus promoting the migration of sodium ions in a sulfur electrode. In addition, the formation of an insulating sulfur layer at the interface between beta-alumina and a sulfur electrode during recharge considerably increases the internal resistance of the battery to stop the recharging before sodium polysulfides are completely converted to sulfur. This limits the use of sulfur as a reactant. Under such circumstances, attempts to impregnate alumina particles into carbon felts have been disclosed.
As an approach to solve the above problems, U.S. Pat. No. 4,084,041 discloses the use of an alumina-treated carbon felt around a beta-alumina tube. According to this approach, sodium polysulfides do not remain unchanged and are oxidized to sulfur when recharged, achieving high utilization efficiency of sulfur as a reactant. GB 1528672 discloses the use of a carbon felt produced by mixing carbon fibers with alumina fibers.
Further, U.S. Pat. No. 4,944,991 discloses the dispersion of alumina in the form of small nodules throughout a carbon felt by impregnating an aqueous solution of an acidic aluminum compound such as aluminum sulfate into the carbon felt. EP 1296392 discloses a method for increasing the charge-discharge efficiency of a battery and reducing the internal resistance of the battery by needle-punching glass fibers into a carbon felt. KR 2012-0075068 discloses a method for improving the performance of a sodium-sulfur battery by partially coating alumina on the outer wall of beta-alumina tube highly wettable with sodium polysulfides in the form of fine semi-spheres. According to this method, the alumina is coated by spraying to maintain a constant reaction area of sodium and sulfur during charge-discharge. In addition, sodium polysulfides are prevented from being attached to the outer wall of beta-alumina and are easily separated from the outer wall of beta-alumina once it has been attached thereto.
However, the conventional methods are succeeded in increasing the utilization efficiency of sulfur as a reactant during long-term cycles but suffer from the problem of increasing the internal resistance of the battery. Other problems of the conventional methods are that the introduction of alumina into carbon felts is complicated, small alumina powder aggregates are simply formed in carbon felts having a porosity of 90% or more, and alumina is not easily coated on the surface of carbon fibers.